Effect of quartz content on the nature of Al-substituted 11 Å tobermorite in hydrothermally treated CaO—Al₂O₃—SiO₂—H₂O systems

Lime-metakaolin-quartz mixtures containing varying amounts of quartz were prepared and hydrothermally cured at 180°C. The nature of hydration products with reaction time was studied using a combination of solid-state²⁷ Al MAS NMR, XRD, DTA-TGA, SEM and wet chemical techniques. Hydrogarnet always preceded Al-substituted 11Å tobermorite irrespective of the amount of quartz used and supplied most, if not all, of the Al to form Al-substituted 11Å tobermorite. The availability of silicate anions from quartz was an important factor in providing a sink for the Al species to be in tetrahedral coordination. It also offered a means for controlling the extent of Al substitution into the 11Å tobermorite crystal lattice with reaction time and the extent of breakdown of the hydrogarnet. The growth of hydrogarnet was inhibited with increasing amounts of quartz, while the formation of Al-substituted 11Å tobermorite was accelerated. With increasing amounts of quartz, the crystallite size in the c-direction of Al-substituted 11Å tobermorite decreased and a change in its morphology was evident from platelets to aggregated petal-like, fine crystals.